Merge pull request #2370 from PeterKietzmann/remove_status_image_README

doc: remove Travis status image in REAMDE

README.md

@@ -26,8 +26,6 @@
 
 The friendly Operating System for IoT!
 
-![Travis CI Status](https://travis-ci.org/RIOT-OS/RIOT.svg?branch=master)
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 ## FEATURES
 RIOT OS is an operating system for Internet of Things (IoT) devices. It is based on a microkernel and designed for
 * energy efficiency